Nearly 2,000 mothers and their babies participated in the study, which found that inadequate vitamin D is linked to poorer mental and movement skills in infants.

“This study is really going to open the door for those of us who have been advocating a stronger stance on vitamin D recommendations for pregnancy and pre-pregnancy,” says Valencia Walker, MD, a neonatologist at Mattel Children’s Hospital UCLA. She reviewed the study for WebMD. “This study helps prove that D matters, and that pregnant women should not be vitamin D deficient.”

For the study, researchers measured the vitamin D levels of pregnant women mainly during their second trimester. At 14 months, their babies underwent an approximately hour-long battery of standardized tests that measured their mental development and their psychomotor skills, or their ability to control their physical movements.

In all, 1,820 mother-infant pairs took part in the study, which is published in the journal Pediatrics.

The research team found that, for both measures, babies whose mothers had an optimal level of vitamin D scored slightly higher than babies of mothers who were D deficient.

Such a small difference in scores may not make any noticeable difference for individual babies, says researcher Eva Morales, MD, PhD, MPH, of the Centre for Research in Environmental Epidemiology in Barcelona.

But because so many women don't get enough vitamin D, those small differences may translate into a large societal impact when the number of babies at risk of poorer development is taken into consideration.

“Further studies are needed to conclude if these scores can predict future neuropsychological development later in life,” Morales says.